A Palm Beach heiress and charity benefactor pleaded guilty Tuesday to using foreign bank accounts to hide more than $43 million from the IRS in one of the largest cases in the continuing U.S. crackdown on offshore tax evasion.

Mary Estelle Curran, 79, will pay nearly $21.7 million in penalties after admitting she filed two years of false tax returns. She failed to disclose her Switzerland and Liechtenstein accounts on federal tax returns from 2001 to 2007, according to a Justice Department announcement.

Curran faces a maximum six-year prison term at a scheduled March 29 sentencing, though a plea agreement filed in court suggested that the applicable guideline level in her case would be 30 months to 37 months.

She must also file amended federal tax returns for 2001 through 2007 and pay $667,716 in overdue taxes, plus interest and penalties.

"U.S. citizens who seek to avoid their tax obligations by hiding income in undeclared bank accounts abroad should by now be fully on notice that they will be held accountable for their actions, both civilly and criminally," said U.S. Attorney Wifredo Ferrer of Florida's Southern District.

The guilty plea and prospect of prison time come in sharp contrast with Curran's life in a five-bedroom, six-bathroom Palm Beach family home, which Realtor.com estimates is worth more than $3 million.

Additionally, a 2011 report in the Palm Beach Daily News shows Curran has given years of support and service to Opportunity Inc. Early Childhood Center, which provides education to children of needy families. She also has been a supporter of the Rehabilitation Center for Children and Adults, a 2010 Palm Beach Daily News report shows.

Curran's attorney, Roy Black, famed for his successful defense of William Kennedy Smith on rape charges in Palm Beach, did not respond to a phone message seeking comment.

Curran is the latest of dozens of offshore banking clients snared in the offshore tax-evasion crackdown launched by the Justice Department and IRS in 2008. She inherited the offshore accounts from her husband, Mortimer. He died in 2000, according to a statement of facts filed Tuesday in her court case.

Court records show some of her assets were held in the names of offshore firms at UBS, a Swiss banking giant that in 2009 agreed to a $780 million settlement of federal criminal charges that it had helped wealthy American clients evade taxes.

In 2008, fiduciaries managing Curran's financial affairs closed the UBS accounts and transferred the assets to a Liechtenstein bank under a corporate name, the court records show.